1. Field of the Invention
The present invention relates to hydraulic drive control systems and, more particularly, to a hydraulic control system which is adapted to operate a plurality of linear and rotational drive assemblies in the injection unit of a plastics injection molding machine.
2. Description of the Prior Art
The high performance levels required of modern injection molding machines, and the need for operational adaptability to a wide variety of conditions make it necessary to generate all the movement phases of the injection cycle by means of hydraulic drive units in the form of linear hydraulic motors, viz. hydraulic cylinder assemblies, and rotary hydraulic motors.
The relatively simple hydraulic control systems of the past are giving way to increasingly sophisticated control systems designed to improve the energy efficiency of the system, while also offering a greater versatility and adaptability to changing operational requirements dictated by the nature of the parts which are to be injection-molded.
It is known that the efficiency of a hydraulic drive system, in terms of the ratio between the energy transmitted and the energy consumed, can be greatly improved by using, instead of a control system with a source of constant pressure and pressure-throttling valves, a control system with a continuously variable pressure source in a load-sensing feedback circuit.
It has therefore already been suggested to use as a pressure source a variable delivery hydraulic pump, the output of which is continuously adjusted by means of a pump adjustment unit which receives pressure feedback signals from the supply line leading to the drive assembly and, in response thereto, adjusts the output pressure of the pump to the pressure level in the supply line.
The flow rate and the pressure in the supply line are adjustable by means of appropriate control valves, a preferred type of valve being the so-called proportional-response valve with an adjustment position transducer which makes it possible to directly feed the desired adjustment values to the drive solenoid of the valve.
The latest improvement which has been suggested in this field relates to the use of so-called proportional P/Q valves which make it is possible to alternatingly control the pressure (P) and the flow rate (Q) in the supply line. A proportional P/Q valve thus takes the place of separate control valves for the pressure program and for the flow rate program.
The proportional P/Q valve operates under a constant, low pressure gradient which is maintained by a spring-biased pump adjustment valve, through the continuous adjustment of the pump output pressure to a level which exceeds the supply line pressure downstream of the proportional P/Q valve by a small pressure drop of constant size, the operational pressure gradient.
A hydraulic control system using a proportional flow control valve is disclosed in the German Pat. No. 31 19 095 and in the corresponding Canadian Pat. No. 1,171,757. Structural and operational details of proportional valves are found in the handbook "Hydraulics: Theory and Applications" (1984) published by the Robert Bosch GmbH, Stuttgart, Germany.
Typically, a hydraulic drive operation involves a displacement phase during which the speed or flow rate represents the limiting factor and a pressure phase during which the force or fluid pressure is controlled. Corresponding programs of flow rate limits and pressure limits are fed to the proportional P/Q valve.
As soon as the supply line pressure downstream of the proportional P/Q valve reaches the pressure limit of the flow rate program, the adjustment operation of the valve automatically converts from following the flow rate program to following the pressure program.
A flow rate program may be fed to the proportional P/Q valve either in a direct control configuration in which the valve settings are verified by an adjustment position transducer of the valve which is coupled to its proportional-response solenoid, or in a feedback control configuration using a feedback circuit with a separate signal source, such as a flow metering device in the supply line, or a displacement-to-voltage converter which is operatively connected to the driven load.
A pressure program is normally fed to the proportional P/Q valve in a feedback control configuration using a feedback circuit with a pressure transducer which is arranged in the supply line downstream of the valve.